1. Field of the Invention
The present invention generally relates to a frequency power supply apparatus under control of pulse-width modulation. More specifically, the present invention is directed to a PWM (pulse-width modulation)-control type power source whose ground potential has no high-frequency (modulation frequency) signal components.
2. Description of the Related Art
In FIG. 1, there is shown a major circuit of a typical PWM controlled power supply apparatus.
A circuit arrangement of this power supply apparatus is as follows: A voltage of a single-phase AC power supply source 501, one line of which is grounded, is rectified in a full waveform rectifying mode by a bridge rectifier 502 constructed of diodes 521 to 524. The rectified voltage is converted into a predetermined DC voltage via a step-up chopper 507 constructed of a DC reactor 571, a switching element 572, and a diode 573, and a smoothing capacitor 503. Thereafter, the resultant DC voltage is again converted into a corresponding AC voltage by way of pulse-width-modulation (PWM) controlling operations by an inverter circuit 504 arranged by a bridge-construction of switching elements 541 to 544. The resultant AC voltage is filtered by an L-type filter 505 including a reactor 51 and a capacitor 552 so as to eliminate high-frequency (modulation) signal components and converted into a smoothed sinusoidal wave. Finally, this sinusoidal wave voltage is applied to a load 506.
The function of the step-up chopper 507 is to increase the input voltage in order that the output AC voltage is equal to the power source voltage. Alternatively, the input AC voltage may be stepped up by, for instance, a transformer and thereafter may be rectified.
In fact, according to the above-described conventional power supply apparatus, the high-frequency (modulation) signal components have been eliminated from the AC voltage applied to the load. However, the high-frequency signal components appear in the ground potentials due to an employment of the PWM control operations.
Referring now to waveforms shown in FIG. 2, a description will be made why the high-frequency (modulation) signal components are contained in the ground potentials.
Assuming that a supply voltage (a voltage across L-N terminals) is equal to "V.sub.1 ", a ground potential "v.sub.N " at a ground side "N" of the power source 501 is equal to zero ("v.sub.N "=0), whereas another ground potential "v.sub.L " at a non-ground side "L" thereof is equal to "V.sub.1 " (v.sub.L =V.sub.1) (see FIG. 2A). A ground potential "v.sub.DN " at a load side "DN" of a DC output from the rectifier 502 is determined by conduction of the diodes 522 and 524. That is, during the positive period of the power source voltage V.sub.1, the ground potential v.sub.DN is equal to zero (v.sub.DN =0) since one diode 24 is turned ON. During the negative period of the power source voltage V.sub.1, the ground potential v.sub.DN is equal to V.sub.1 (v.sub.DN =V.sub.1). As a result, if the DC voltage is "E.sub.o ", another potential voltage v.sub.DP at the positive side "DP" is determined by: EQU v.sub.DP =v.sub.DN +E.sub.o,
where E.sub.o is nearly equal to a constant (see FIG. 2B).
A ground potential "v.sub.V " at an AC output "V" phase of the inverter circuit 504 is determined by turning ON/OFF the switching elements 543 and 544. When one switching element 543 is turned ON, the ground potential V.sub.v is equal to v.sub.DP (v.sub.V =v.sub.DP), whereas when the other switching element 44 is turned ON, the ground potential v.sub.V is equal to v.sub.DN (v.sub.V =v.sub.DN). In general, all of these switching elements are turned ON/OFF at a high speed (e.g., 10 to 20 KH.sub.Z) under PWM control so that the ground potential of the V phase "v.sub.V " contains the high-frequency signal (noise) components as "v.sub.DP " and "v.sub.DN " being envelope lines because of the PWM control operation (see FIG. 2C).
Another ground potential "v.sub.U " at an U phase of the AC output is determined by the following equation, if the AC output voltage is equal to "V.sub.o ": EQU v.sub.U =v.sub.V +V.sub.o.
As a consequence, in the case when the AC output voltage V.sub.o is equal to the power source voltage V.sub.1 and also has a in-phase condition thereto, the ground potential "v.sub.U " is represented by a waveform shown in FIG. 2D. This ground potential "v.sub.U " contains the high-frequency signal components similar to the above-described ground potential "v.sub.V ". A maximum value "v.sub.U (max)" of this ground potential "v.sub.U " is defined by: EQU v.sub.U (max)=a peak value of V.sub.o +E.sub.o.
It is obvious that this maximum value "v.sub.U (max)" is considerably higher than the AC output voltage V.sub.o from the inverter 504.
In the case when the AC output voltage V.sub.o is equal to the power source voltage V.sub.1 and also has a reverse-phase condition thereto, the ground potential "v.sub.U " is represented by a waveform shown in FIG. 2E. This ground potential "v.sub.V " also contains the high-frequency signal components.
While there has been described above, a system according to the conventional PWM controlled power supply apparatus, since the variations in the ground potentials at the output terminal thereof contain the high-frequency signal (noise) components produced by the high speed switching operation of the DC/AC inverter, it is necessary to employ a large-scale line filter so as to filter out such high-frequency (modulation) noise components. In particular, as a power supply apparatus used for computers, the high-frequency noises must be completely eliminated. Also, in the case when a surge suppressor capable of absorbing indirect lightings or switching surges is provided between the line and ground, this surge suppressor may be burned out. Specifically, since the very high voltage is instantaneously applied, the rated voltage of the surge suppressor must be selected to be a proper high value.